Get announce react to network and services coming and going

[probonopd]

e.g., by watching /proc
or
netstat -lntu
or
http://wiki.openwrt.org/doc/techref/ubus or hotplugorprocd_add_mdns_service` <-- OpenWrt; sounds interesting; undocumented?

The dropbear init script of a forked OpenWrt was extended by
[ "${mdns}" -ne 0 ] && procd_add_mdns "ssh" "tcp" "$Port" "daemon=dropbear"
perhaps this is the route to go? But then all init scripts of services would have to be changed. Where is the documentation for the mdns OpenWrt package?

/etc/services on OpenWrt contains a translation table from ports to services; we could use that too, maybe in conjunction with something like netstat -altuen | grep LISTEN or parsing /proc/net/tcp and /proc/net/udp.

Pull requests are welcome.